In an engine or other prime mover with a piston or other reciprocating part, for example, the piston and a cylinder may wear due to sliding motions between them, resulting in production of iron powder or other magnetic material which is entrained in drain oil from the engine flowing through a passage. Thus, it is required to measure a concentration of the magnetic material contained in order to accurately grasp worn degree of the equipment.
Generally, in order to grasp the worn degree of the equipment, lubricant or drain oil is manually sampled to measure a concentration of magnetic material; alternatively, a magnetic material concentration measuring device is arranged adjacent to the passage for the lubricant or drain oil to measure the concentration of magnetic material.
A magnetic material concentration measuring device comprises, for example, an observational LC oscillation circuits with a first coil adjacent to a passage for drain oil and a corrective LC oscillation circuit with a second coil arranged in a position not affected by the magnetic material in the drain oil, the concentration of the magnetic material being detected on the basis of difference in oscillating frequency between the LC oscillation circuits (see, for example, Patent Literature 1). Another example comprises magnetic-field applying means and magnetic measuring means with a magnetic sensor in the form of a superconducting quantum interference device both arranged adjacent to a passage for drain oil so as to detect only magnetic fields of magnetized components, which has been disclosed for example in the latter:    [Patent Literature 1] JP 2005-83897A    [Patent Literature 2] JP 10-268013A